Fix sprockets.gemspec to include MIT-LICENSE

The original License should be included in all packages of sprockets. The current gemspec does so by including "LICENSE"-File which doesn't exist. This commit fixes that referencing the "MIT-LICENSE"-File.

sprockets.gemspec

@@ -8,7 +8,7 @@ Gem::Specification.new do |s|
   s.description = "Sprockets is a Rack-based asset packaging system that concatenates and serves JavaScript, CoffeeScript, CSS, Sass, and SCSS."
   s.license = "MIT"
 
-  s.files = Dir["README.md", "CHANGELOG.md", "LICENSE", "lib/**/*.rb"]
+  s.files = Dir["README.md", "CHANGELOG.md", "MIT-LICENSE", "lib/**/*.rb"]
   s.executables = ["sprockets"]
 
   s.add_dependency "rack",            "> 1", "< 3"